For which the following is a specification:
The present invention is a heat curable silicone rubber composition with high resistance to degradation by engine oils and coolants, and in particular, resistance to synthetic engine oils and long-life engine coolants.
Gaskets and packing materials formed from silicone rubber frequently suffer from poor resistance to hot hydrocarbon oils, for example, engine oil and gear oil, and from a poor resistance to radiator coolants. As a consequence, oil and coolant leaks may develop during the long-term use of silicone rubber as gasket materials in such applications.
Synthetic engine oils have as major components poly-alpha-olefins and esters which may break down to acids. Synthetic engine oils may also contain lesser amounts of additives such as oxidation inhibitors, rust inhibitors, anti-wear and extreme pressure agents, friction modifiers, detergents and dispersants, pour-point depressants, viscosity improvers, and foam inhibitors. These components of synthetic oils may interact with silicone rubber differently than do hydrocarbon oils, adversely impacting the sealing properties of the rubber.
Similarly, extended-life coolants may contain organic acids, such as aliphatic monobasic acids, hydrocarbyl dibasic acids, and the alkali metal, ammonium or amine salts of monobasic acids or hydrocarbyl dibasic acids as components that may interact with silicone rubber, in addition to conventional additives such as ethylene glycol, water, and corrosion inhibitors. See for example, U.S. Pat. No. 4,647,392 to Darden, et al. The acids and salts can attack the silicone rubber.
Fluorosilicone rubbers are generally known in the art for their resistance to fuel, oil, chemicals, and solvents. However, fluorosilicone rubbers are relatively costly materials, and not considered to be cost effective in many applications involving contact with engine oils and coolants. Therefore, there is a need to improve the performance of non-fluorinated silicone rubbers in contact with engine oils and coolants.
Inoue et al., in U.S. Pat. No. 4,689,363, teach compositions for room-temperature curable silicone rubber that are resistant to conventional engine oils. The compositions comprise 100 parts by weight of a hydroxy end-terminated polydiorganosiloxane having a linear molecular structure; from 1 to 25 parts by weight of an organosilicone having, in each molecule, at least two hydrolyzable groups bonded to the silicon atom or atoms; and from 1 to 50 parts of an alkali metal salt of a weak acid having a pKa in the range from 2.0 to 12.0 at 25xc2x0 C. The polyorganosiloxane has a viscosity in the range of 25 to 500 Paxc2x7s or preferably from 1 to 100 Paxc2x7s at 25xc2x0 C.
Koshii et al., in U.S. Pat. No. 5,013,781, teach compositions for room-temperature curable silicone rubber resistant to conventional coolants and hydrocarbon oils. A polyorganosiloxane composed of Rxe2x80x23SiO0.5 and SiO2 units or Rxe2x80x23SiO0.5, R2xe2x80x2SiO and SiO2 units is included at 1 to 50 weight parts in a composition containing 100 parts polydiorganosiloxane, 5 to 300 weight parts inorganic filler, 0.1 to 10 weight parts alkoxysilane adhesion promoter, and a ketoxime silicon compound crosslinker. Koshii et al. teach that the polyorganosiloxane functions in combination with the alkoxysilane adhesion promoter to improve the hydrocarbon oil and coolant (chemical) resistance of room-temperature curable silicone rubber. The molar ratio of the Rxe2x80x23SiO0.5 to SiO2 in the polyorganosiloxane must be from 0.5:1 to 1.5:1. The polydiorganosiloxane is a flowable polymer, and has a viscosity within the range of 0.0001 to 0.1 m2/s at 25xc2x0 C., and the chain terminals contain a silicon-bonded hydroxyl group or a silicon-bonded hydrolyzable group.
The approaches by Inoue et al. and Koshii et al. do not address the need for silicone compositions that are used in contact with synthetic engine oils or extended-life coolants. Furthermore, while these approaches are useful for room-temperature-vulcanizable compositions, they do not address the additional need for heat-curable silicone compositions with improved resistance to coolants and oils. Heat curable silicone rubbers are used in applications that experience much higher stress, and may possibly be exposed to higher temperature and pressure and harsher chemical environments. For example, a heat curable silicone rubber may be used in applications requiring tensile strengths of from about 60 to 106 kg/cm2, while room-temperature-vulcanizable compositions are more typically useful at lower tensile strengths from about 10 to 35 kg/cm2. More particularly, heat cured silicone rubber is used in engine and coolant system applications where such systems are under heat or pressure, and properties such as compression set and compression stress relaxation are of concern. Furthermore, heat curable silicone rubbers often have larger cross-sectional areas exposed to chemical agents, compared to room-temperature-vulcanizable silicone compositions, which are typically used in thin gaskets. This additional cross-sectional area exposes more surface to chemical attack. Further, as gaskets are exposed to temperature cycling, they tend to swell in oil or coolant when heated, and shrink when cooled. Thus, increased cross-sectional area of heat-cured silicone rubber gaskets significantly increases the chemical exposure of the entire gasket.
Therefore, a non-fluorinated, heat-curable silicone rubber composition is needed that is resistant to standard and long-term engine coolants, and standard hydrocarbon and synthetic motor oils.
This invention is a heat-curable silicone rubber composition comprising:
(A) 100 parts by weight of an organosiloxane polymer base comprising an organosiloxane polymer containing at least 2 silicon-bonded alkenyl groups in each molecule and about 1 to 65 weight percent reinforcing silica filler,
(B) curing component sufficient to cure the composition when heated, and
(C) an effective amount of at least one metal salt additive selected from the group consisting of
monobasic alkali metal phosphates, alkali metal oxalates, alkali metal tartrates, alkali metal tetraborates, alkali metal phthalates, and alkali metal citrates;
dibasic metal phosphates where the metal is selected from the group consisting of sodium, potassium, calcium and magnesium;
metal acetates, where the metal is selected from the group consisting of sodium, potassium, calcium, and magnesium;
metal sulfates where the metal is selected from the group consisting of sodium, potassium, calcium, magnesium, aluminum, and zinc; and
metal carbonates where the metal is selected from the group consisting of sodium, potassium, calcium, magnesium, aluminum and zinc.
The compositions of this invention provide superior compression set and compression stress relaxation results over other silicone compositions.
Component A, the organosiloxane polymer base (the base), comprises a mixture of an organosiloxane polymer with reinforcing silica filler. The organosiloxane polymer in the base has the average composition of RaSiO(4-a)/2. In the formula R is selected from substituted and unsubstituted monovalent hydrocarbon groups and is exemplified by alkyl groups such as methyl, ethyl, and propyl; alkenyl groups such as vinyl, allyl, butenyl, and hexenyl; aryl groups such as phenyl; and aralkyls such as 2-phenylethyl. The subscript a is a value from 1.95 to 2.05.
The organosiloxane polymer has at least 2 silicon-bonded alkenyl groups in each molecule. The alkenyl groups can be bonded in pendant positions, at the terminal positions, or at both positions. The molecular structure of the organosiloxane polymer generally has a degree of polymerization (dp) in the range of from 200 to 20,000, and preferably has a dp in a range of 1000 to 20,000. This dp range includes polymers which are thick, flowable liquids as well as those that have a stiff, gum-like consistency. The organosiloxane polymer can be a homopolymer or a copolymer or a mixture of such polymers. The siloxy units comprising the organosiloxane polymer are exemplified by dimethylsiloxy, vinylmethylsiloxy, and methylphenylsiloxy. The molecular terminal groups in the organosiloxane polymer are exemplified by trimethylsiloxy, and vinyldimethylsiloxy groups. The organosiloxane polymer is exemplified by vinyldimethylsiloxy-endblocked dimethylsiloxane-vinylmethylsiloxane copolymer, vinyldimethylsiloxy-endblocked polydimethylsiloxane, vinylmethylhydroxysiloxy-endblocked dimethylsiloxane-vinylmethylsiloxane copolymer, and vinyldimethylsiloxy-endblocked dimethylsiloxane-methylphenylsiloxane-vinylmethylsiloxane copolymer. A preferred polymer is a vinyldimethylsiloxy-terminated polydimethylsiloxane gum comprising 0.142 mole percent of vinylmethylsiloxane units and exhibiting a plasticity of 55-65 mils based on ASTM D926.
The base also contains a reinforcing silica filler, to provide increased mechanical properties in the present heat cured silicone rubber composition. The filler can be any silica filler which is known to reinforce polydiorganosiloxane-and is preferably selected from finely divided, fumed and precipitated forms of silica and silica aerogels having a specific surface area of at least about 50 m2/g, and preferably 150 to 400 m2/g. The filler is typically added at a level of about 1 to 65 weight percent of the organosiloxane polymer base, and preferably in a range of 10 to 65 weight percent of the base.
It is preferred to treat the reinforcing silica filler to render its surface hydrophobic, as typically practiced in the silicone rubber art. This can be accomplished by reacting the reinforcing silica filler with a liquid organosilicon compound which contains silanol groups or hydrolyzable precursors of silanol groups. Compounds that can be used as filler treating agents, also referred to as anti-creping agents or plasticizers in the silicone rubber art, include such ingredientsas low molecularweight liquid hydroxy- or alkoxy-terminated polydiorganosiloxanes, including xcex1,xcfx89-silanediols, hexaorganodisiloxanes, cyclodimethylsiloxanes and hexaorganodisilazanes.
In addition to the organosiloxane polymer and reinforcing silica filler, the organosiloxane polymer base may also contain other additives, for example heat stability additives, anti-structure agents, pigments, and extending and semi-reinforcing fillers. Examples of additives include diatomaceous earths, ground quartz, zinc oxide, calcium carbonate, titania, and magnesium oxide. The proportion of such additional fillers will depend on the physical properties and other characteristics desired in the elastomer. Generally such additional fillers can be present in a proportion of from about 10 to 150 parts by weight of the organosiloxane polymer.
The curing component (B) can be any of the well-known curing systems known in the silicone elastomer art. For example, the curable silicone elastomer compositions of this invention may be cured to the elastomeric state by exposure to electron beams, ultraviolet rays, electromagnetic waves, or heat. Where heat is used as the curing mechanism, an organic peroxide curing agent may be used. Examples of suitable organic peroxide curing agents include 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane,2,2-vis(t-butylperoxy)-p-diisopropylbenzene, 1,1,bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3, di-t-butylperoxide, benzoyl peroxide, p-chlorobenzoyl peroxide, dicumyl peroxide, tertiary butyl peracetate, tertiary butyl perbenzoate, monochlorobenzoyl peroxide, 2,4-dichlorobenzoyl peroxide, and tertiary butyl cumyl peroxide. A preferred organic peroxide curing agent is 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.
Another heat curing system which is applicable is where the curable silicone elastomer composition is cured by crosslinking the polyorganosiloxane with an organohydrogensiloxane crosslinker in the presence of a platinum group metal-containing catalyst. The organohydrogensiloxane crosslinker can contain an average of at least two silicon-bonded hydrogen atoms per molecule, and no more than one silicon-bonded hydrogen atom per silicon atom, the remaining valences of the silicon atoms being satisfied by divalent oxygen atoms or by monovalent hydrocarbon radicals comprising one to seven carbon atoms. The monovalent hydrocarbon radicals can be, for examples, alkyls such as methyl, ethyl, propyl, tertiary butyl, and hexyl; cylcoalkyls such as cyclohexyl; and aryls such as phenyl and tolyl. The platinum group metal-containing catalyst can be any such catalyst which is known to catalyze the reaction of silicon-bonded hydrogen atoms with silicon-bonded vinyl groups. By platinum group metal, it is meant ruthenium, rhodium, palladium, osmium, iridium, and platinum.
Component (C) is an effective amount of a metal salt additive selected from at least one of the group consisting of monobasic alkali metal phosphates; alkali metal oxalates; alkali metal tartrates; alkali metal tetraborates; alkali metal phthalates; alkali metal citrates; dibasic metal phosphates where the metal is selected from the group consisting of sodium, potassium, calcium and magnesium; metal acetates, where the metal is selected from the group consisting of sodium, potassium, calcium, magnesium; metal sulfates where the metal is selected from the group consisting of sodium, potassium, calcium, magnesium, aluminum, and zinc; and metal carbonates where the metal is selected from the group consisting of sodium, potassium, calcium, magnesium, aluminum and zinc.
Without wishing to be bound to any particular theory, the inventors propose that the metal salt additives imparting resistance to coolants and the metal salt additives imparting resistance to synthetic oil in the present invention act in a manner analogous to salts that are present in an aqueous buffer system. Aqueous buffer systems are well known in chemical arts, and are characterized by their ability to resist pH changes when diluted or when various amounts of acid or base or added. One reference describing how buffer systems work is Peters, Hayes, and Hieftje, xe2x80x9cAqueous Acid-Base Reactions,xe2x80x9d Chemical Separation and Measurements, W. B. Saunders Company, (1974) pp. 100-112. Silicone rubber is more stable in a chemical environment where the pH of a system is at neutral or near-neutral. In the present invention, metal salts which, in an aqueous system, are used in buffer solutions that maintain a pH in a range of about 3 to 8 may be useful.
For water-soluble metal salt additives, a pKa between 3 and 8 is useful. For metal salt additives with limited solubility, the salt should be capable of increasing the pH of a dilute aqueous acidic solution from a pH below 3 to a pH between about 3 and about 8, or from greater than 8 to between 3 and 8.
By effective amount of a metal salt additive it is meant an amount that provides a heat curable silicone rubber composition retaining greater than 35 percent of the sealing force when exposed to long life coolant for 6 weeks in a compression stress relaxation (CSR) test, or retaining more than 10 percent of sealing force and having a compression set of at most 40 percent after 6 weeks in synthetic motor oil. Compression stress relaxation is a described below in the examples. A typical effective amount will be in the range of about 0.5 to 20 weight parts salt for 100 weight parts organosiloxane polymer base.
When resistance to long life coolants is desired, the metal salt additive is preferably an alkali metal monobasic salt selected from the group consisting of phosphates, oxalates, tartrates, tetraborates, phthalates, citrates, acetates, sulfates, and carbonates. The alkali metal of such salt may be sodium or potassium, though not limited to these. A preferred alkali metal monobasic salt for this purpose is monosodium phosphate. When monosodium phosphate, NaH2PO4, is used, a useful amount is about 0.5 to 7.5 parts by weight monosodium phosphate per 100 parts by weight of the organosiloxane base. Amounts below about 0.5 parts by weight of monosodium phosphate per 100 weight parts organosiloxane base may give less than the desired resistance to the extended life coolant. Surprisingly, the effectiveness is reduced for amounts over about 7.5 parts by weight monosodium phosphate per 100 weight parts organosiloxane base.
When resistance to synthetic motor oil is desired, the metal salt additive is preferably a dibasic metal salt selected from the group consisting of dibasic metal phosphates where the metal is selected from the group consisting of sodium, potassium, calcium and magnesium; dibasic alkali metal oxalates; dibasic alkali metal tartrates; dibasic alkali metal tetraborates; dibasic alkali metal phthalates; dibasic alkali metal citrates; metal acetates, where the metal is selected from the group consisting of sodium, potassium, calcium, magnesium; metal sulfates where the metal is selected from the group consisting of sodiurn, potassium, calcium, magnesium, aluminum, and zinc; and metal carbonates where the metal is selected from the group consisting of sodium, potassium, calcium, magnesium, aluminum and zinc. A preferred salt is disodium phosphate, Na2HPO4. When disodium phosphate is used a preferred amount is in a range from about 0.5 to 20 weight parts per hundred weight parts organosiloxane base. Concentrations of disodium phosphate below about 0.5 weight parts per 100 parts of organosiloxane base do not provide the desired resistance to motor oil. At concentrations above about 20 parts the physical properties of the heat-curable elastomer are not desirable.
A combination of metal salt additives can be used to provide resistance to both coolants and oils. Where a combination of metal salt additives is used, the total amount of the salts should not exceed 20 parts of the combined weight of metal salt additives per hundred weight parts of organosiloxane polymer base. As above, the physical properties of the heat curable silicone elastomer are not desirable above about 20 weight parts of salt in the organosiloxane polymer base.
The compositions of the present invention may be prepared by any convenient procedure. The organosiloxane polymer base and the additive (C) may be mixed together with any additional fillers or other ingredients, with sufficient heat and shear to form a uniform base. In a preferred method, a high consistency organosiloxane polymer base is formed first, by mixing the organosiloxane polymer with any additional fillers or other ingredients, and then the metal salt additive (C) is incorporated into the base, using a mixing device, such as a two-roll mill.